This PR is the 6ᵗʰ step in the journey to solve issue #136139 and make the entire Flutter repo more readable.

(previous pull requests: #139048, #139882, #141591, #142279, #142634)

The current focus is on `packages/flutter/lib/src/material/`. The previous 2 PRs covered files in this directory starting with letters `a-m`; this one takes care of everything else.